1. Field of Invention
The present invention relates to a composite membrane which is fabricated by depositing an inorganic ion-conducting thin film on a cation-selective organic polymer membrane substrate using Pulse Laser Deposition (PLD) or reactive magnetron sputtering. Furthermore, the present invention relates to various electromembrane systems incorporating such membranes to improve their performance. In particular, these membranes are useful in electrolysis and bipolar membrane electrodialysis systems for the production of sodium hydroxide and acid from solutions of alkali metal salts.
2. Description of the Prior Art
Cation-selective organic polymer membranes are used in a variety of applications in the biological, medical, chemical, food, pulp and paper and other industries. In particular, such membranes are used in electrolytic systems (e.g. chloralkali cells for the production of chlorine and sodium hydroxide from sodium chloride), electrodialysis systems (e.g. desalination of brackish and sea water), bipolar membrane electrodialysis systems (e.g. splitting of the salts of organic acids into sodium hydroxide and organic acid) and fuel cells. Examples of such membranes are: Nafion (a trademark of E.I DuPont de Nemours, Wilmington, Del., USA), Tokuyama Soda Neosepta (trade-mark) CMX, CM-1 and CM-2, Asahi Glass Selemion (trade-mark) CMV and CSV and Raipore R-4010 and R-1010 (trade-marks of RAI Research Corporation, Hauppauge, N.Y., USA). New applications involving organic polymeric cation-selective membranes are constantly being developed (Paleologou, M. and Berry, R. M., Electrodialytic Water-Splitting Process for the Treatment of Aqueous Electrolytes, U.S. Pat. No. 5,006,211, Apr. 9, 1991; Paleologou, M., Wong, P-Y., Berry, R. M., A Solution to Caustic/Chlorine Imbalance: Bipolar Membrane Electrodialysis, J. Pulp Paper Sci., 18, J138 (1992).
A typical unit electrolysis cell employs two electrodes, an anode and a cathode, with a cation-selective membrane between them. In a particular application, sodium sulphate is fed into the anode compartment and water (or a dilute sodium hydroxide solution) is fed into the cathode compartment. When a voltage is applied between the two electrodes, the sodium ions migrate through the membrane towards the negative electrode where they combine with hydroxide ions, generated from the reduction of water at the cathode, to produce sodium hydroxide. The migrating sodium ions in the anode compartment, are replaced by hydrogen ions, generated by the oxidation of water at the anode, to produce sulphuric acid. Thus, the product from the anode compartment is acidified sodium sulphate, and the product from the cathode compartment is sodium hydroxide. To reduce capital and operating costs, bipolar membranes can be incorporated into such a system in an alternate arrangement with cation-selective membranes, in which case, it is referred to as a bipolar membrane electrodialysis (BME) system (Paleologou, M., Wong, P-Y., Berry, R. M., A Solution to Caustic/Chlorine Imbalance: Bipolar Membrane Electrodialysis, J. Pulp Paper Sci., 18, J138 (1992). The generation of new products (acid and base) distinguishes BME from conventional electrodialysis (ED), which simply employs alternate cation- and anion-selective membranes in between two electrodes for the concentration and/or dilution of salt solutions. The low capital and operating costs associated with BME and ED, as compared to electrolysis, are due to the stacking of numerous unit cells in between two electrodes of small area.
At present, a variety of inorganic ion-selective membranes made of solid state ionic conductors are known (The Principles of Ion Selective Electrodes and of Membrane Transport, W. E. Morf, Ed., Chapter 10, Elsevier Pub., Co., Amsterdam, 1988; Ion-Selective Electrode Methodology, A. K. Covington, Ed., Chapter 9, CRS Press, Boca Raton, 1979). Such materials include metal super ion conducting materials (MESICON) suitable for the fabrication of ceramic ion-conducting membranes with high ion conductivity at low temperature, high selectivity for alkali metal ions and comparative stability in water and corrosive media (Balagopal, S. H., Gordon, J. H., Virkar, A. V., Joshi, A. V. Selective Metal Cation-conducting Ceramics, U.S. Pat. No. 5,580,430, Dec. 3, 1996). Among them, the three-dimensional framework fast ion conductors of the family NASICON (Hong, H. Y-P., Crystal Structures and Crystal Chemistry in the System Na.sub.1+4 Zr.sub.2 Si.sub.x P.sub.3-x O.sub.12, Mat. Res. Bull., 11, 173, 1976) have been studied extensively and found to be appropriate for the fabrication of ion-selective membranes (Fabry, P., Huang, Y. L., Caneiro, A., Patrat, G., Dip-coating Process for Preparation of Ion-sensitive NASICON thin films, Sensors and Actuators, B6, 299, 1992; Damasceno, O., Siebert, E., Khireddine, H., Fabry, P., Ionic Exchange and Selectivity of NASICON Sensitive Membranes, Sensors and Actuators, B8, 245, 1992). The polymer membranes have the advantage of being more flexible than the inorganic membranes and, therefore, easier to use in electromembrane cells. However, they exhibit lower ion conductivity and selectivity, and they can be fouled by multivalent metal ions.
An electrolytic approach for the production of sodium hydroxide using a thick ceramic membrane of Nasicon coated by a polymer film of Nafion was previously demonstrated (Joshi, A., Liu, M., Bjorseth, A. and Renberg, L., NaOH Production from Ceramic Electrolytic Cell, U.S. Pat. No. 5,290,405, Mar. 1, 1994). The main disadvantage of this ceramic/polymer membrane is that the ceramic is the substrate, with a thickness of 1.5 mm, on top of which the polymer film is deposited. Such structures are expected to lead to several operational problems in electromembrane systems: (i) the thick ceramic substrate is not very flexible leading to leaking from electromembrane cells, and (ii) the ion fluxes through the membrane are reduced due to the thickness of the ceramic membrane, and (iii) the voltage drop across such membranes is rather high leading to increased energy costs. The operation of electromembrane systems usually involves current densities in the range of 0.1 to 1 A cm.sup.-2. To maintain current densities from 0.1 to 1 A cm.sup.-2, a 1-mm thick inorganic and, in particular, ceramic membrane must have a conductivity of the order of 0.1 Scm.sup.-1 at room temperature. There is no ion conducting ceramic of 1-mm thickness which is able to provide such high conductivity at room temperature. Nasicon (Na.sub.1+x Zr.sub.2 Si.sub.x P.sub.3-x O.sub.12, where 0&lt;x&lt;3) is one of the best fast ionic conductors. At 300.degree. C. this material, for x=2, exhibits a conductivity of 0.35 Scm.sup.-1 and, at room temperature, the conductivity decreases to 10.sup.-3 Scm.sup.-1. At room temperature, an ion current density of the order of 0.1 to 1 Acm.sup.-2 can pass through a Nasicon membrane if the thickness of the membrane is less than 1 .mu.m (1000 .ANG.).
Mesicon and Nasicon materials have generally been produced as bulk materials. Thin films can be produced by various physical vapor deposition methods such as evaporation. However, in the case of Mesicon and, in particular, Nasicon materials, these techniques lead to a loss of the film stoichiometry. In order to produce thin films with good stoichiometry new methods have had to be developed. In recent years, PLD has emerged as one of the most suitable techniques for the deposition of inorganic and, in particular, ceramic materials with complex stoichiometry such as high T.sub.c superconductors (Chrisey, D. B. and Inam, A., MRS Bulletin, XVII, No. 2, 37, 1992). Another promising technique for the deposition of ceramic thin films on various substrates is reactive magnetron sputtering (Handbook of Sputter Deposition Technology, K. Wasa and S. Hayakawa, Noyers Publications, New Jersey, 1992, pp. 81-123). Since solid ionic conductors such as Nasicon are suitable ceramic materials, these techniques were used to deposit thin films of Nasicon on polymeric materials.